I don't want to be seen to advertise, but there is a school in California who will train you for the FAA IR (indeed the full FAA CPL/IR if you want) to JAA standards. They should know, their head instructor went the FAA route, got the FAA ATP, then came back to the UK and converted to JAA fATPL and JAA FI (they also do FAA training in the UK). Becasue they have done this route, they'd be best to advise you on it, and I think they are setting up links with various FTOs in the UK specifically to undertake conversions.
PM me if you want any more contact details.
Rgds
EA